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Seafloor Features and Characteristics of the Black Ledges Area, Penobscot Bay, Maine, USA

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Seafloor Features and Characteristics of the Black Ledges Area, Penobscot Bay, Maine, USA Journal of Coastal Research SI 36 333-339 (ICS 2002 Proceedings) Northern Ireland ISSN 0749-0208 Seafloor Features and Characteristics of the Black Ledges Area, Penobscot Bay, Maine, USA. Allen M. Gontz, Daniel F. Belknap and Joseph T. Kelley Department of Geological Sciences, 111 Bryand Global Science Center, University of Maine, Orono, Maine, USA04469-5790. [email protected], [email protected], [email protected] ABSTRACT The Black Ledges, a series of islands, shoals, and ledges in East Penobscot Bay, Maine, was mapped with digital sidescan sonar and shallow marine seismic reflection equipmentA total of 38 km2 of sidescan and 600 km of seismic data was collected during four cruises in 2000-2001. The sidescan sonar reveals a surficial geology dominated by muddy sediments with frequent, patchy outcrops of gravel and minor amounts of bedrock. There are seven large concentrations of pockmarks with populations totaling over 3500 in the areas of muddy sediments. Generally circular, pockmarks range in size from five to 75 meters in diameter and up to eight meters deep. Calculations show over 2 x 106 m3 of muddy sediment and pore water were removed from the system during pockmark formation. Seismic data reveal a simple stratigraphy of modern mud overlying late Pleistocene glaciomarine sediment, till and Paleozoic bedrock. Seismic data indicate areas of gas-rich sediments and gas- enhanced reflectors in close association with pockmarks, suggesting methane seepage as a cause of pockmark formation. Pockmarks are alsorecognized in areas lacking evidence of subsurface methane accumulations adding further validity to the late stage of development for the field. Elliptical pockmarks, found in nearly 40 m of water, show modification by currents and degradation of the pockmark form. This suggests depletion of methane and a late stage of development. A more intensive investigation of the area, including coring and high-resolution geophysics is currently in progress. ADDITIONALINDEXWORDS: Pockmarks, sidescan sonar INTRODUCTION Pockmarks release methane into the water column and Habitat mapping is currently an area of intense focus by potentially into the atmosphere. Methane is a potent the United States Geological Survey, National Oceanic and greenhouse gas. It traps infrared radiation up to 21 times Atmospheric A s s o c i a t i o n ’s National Marine Fisheries more readily than carbon dioxide (LAMMERS et al., 1995). Service, and state-level equivalents. In order to properly Not only does this release have the potential to alter the assess seafloor habitat, knowledge of the surficial geology global climate system, but it also provides a readily is essential. Geophysical tools such as sidescan sonar and available source of labile carbon for biological activity. The shallow seismic reflection profiling permit the necessary contribution of the marine system to the global carbon cycle observations. is unquantified (LAMMERS et al., 1995). Clearly, a better Penobscot Bay, a large, productive estuary in the Gulf of understanding of these features, the relationships between Maine has been a focus area for such mapping (Figure 1). biological and physical processes, and their effects on the Previous studies in Penobscot Bay, specifically the sub- marine system is needed. embayment of Belfast Bay, have revealed a dynamic The Black Ledges area, the focus of this study, is also seafloor environment dominated by football stadium-sized within Penobscot Bay, but sufficiently removed from features known as pockmarks. KING and MACLEAN Belfast Bay (Figure 1) to be influenced by different water (1970) first described these features on the Scotian Shelf. masses and current regimes (XUE et al., 1999). Workers They are believed to form from pressurized pore fluid from the Maine Geological Survey and the Department of escape (HOVLAND and JUDD, 1988). While these Geological Sciences at the University of Maine collected features are best studied in one part of Penobscot Bay, they several reconnaissance lines in the area between 1984 and also occur throughout the region (BARNHARDT a n d 1999. These few lines showed a potential for pockmarks KELLEY, 1995; FADER 1991). and prompted a more intensive investigation. Journal of Coastal Research, Special Issue 36, 2002 Sea-Floor Features, Maine 334 Figure 1. Location of Penobscot Bay. The large box in the center of the figure locates Penobscot Bay. The inset map shows the location of Maine’s coast within the Gulf of Maine region. Penobscot Bay is the largest estuary along Maine’s coast. Box A shows location of the Black Ledges area, the focus of this study and Box B shows the location of Belfast Bay, which also hosts pockmarks. NH is New Hampshire, NB is New Brunswick, Canada. This paper reports on the first portion of a major research August 2000 and Applied Acoustics Engineering’s digital effort focused on shallow-water, estuarine pockmarks. This system in later cruises), were towed simultaneously. portion involves baseline surficial and subsurface mapping Sidescan sonar data were collected at 100kHz and 500kHz of a pockmarked area and the surrounding seafloor to and seismic data were collected at 700-2000 Hz. ATriton- identify features and characteristics of the area. T h e Elics International topside processing unit was used to ultimate goal is to determine the source of methane, the acquire and post-process the data for both digital systems. origin and age of the source materials, their role in sea-level Sidescan data were collected and processed with Isis changes, potential mechanisms for formation, life cycles, Sonar v4.54 and Delph Map. All data were slant-range e ffect on the global climate system, and effects on corrected. Digital seismic data were collected and sedimentation and seafloor habitat. In this report we processed with Delph Seismic and Seismic GIS. Analog present new data from a series of newly discovered seismic data were analyzed without computer technology. pockmark fields. More importantly, we develop evidence Surficial and subsurface mapping and spatial analysis were for a model of pockmark evolution, including, for the first conducted with A r c View and ArcInfo geographic time, the late stages of development and senescence. information systems software. Navigation was supplied via differential global positioning system and converted to METHODS Universal Transverse Mercator projection for use. The Capn, a digital navigation package produced by Nautical The seafloor was mapped with sidescan sonar and Technologies, was used for cruise planning, cruise shallow seismic reflection profiling equipment during four navigation, autopilot interface, and 30-second position daylong cruises, three in August 2000 and one in January logging. 2001, in the Black Ledges area. The two systems, an The shallow water (<40 m) allowed for collection of Edgetech DF1000 digital, dual-frequency sidescan sonar sidescan data at 100 m range. The fish was towed about towfish and a surface-towed boomer seismic system four to six meters deep at between four and six knots. (Ocean Research Engineering’s analog GeoPulse during Journal of Coastal Research, Special Issue 36, 2002 Gontz, Belknap and Kelley 335 RESULTS Natural gas, methane, is found within the section at many places. It occurs as areas of large-scale acoustic wipeout or A total of 38 km2 of seafloor was imaged with sidescan gas-enhanced reflectors (Figure 4) (YUAN et al., 1992; sonar and over 600 km of seismic data were collected F L O O D G ATE and JUDD, 1992; FADER, 1991). during four cruises. A complete sidescan sonar mosaic was Stratigraphically, the methane occurs within the Holocene created for the area (Figure 2). muds or along the Holocene-Pleistocene unconformity. A stratigraphic framework of Penobscot Bay was Pockmarks have been observed over areas of acoustic developed from seismic data and supported with sediment wipeout and gas-enhanced reflectors and both indicators of cores. (BARNHARDT et al., 1997; BELKNAP and SHIPP, methane have been observed without associated pockmarks. 1991; KNEBEL and SCANLON, 1985). Seismic records The enhanced reflectors suggest methane is migrating along show acoustic basement as bedrock, overlain by till, the unconformity. Methane is hypothesized to be the fluid glaciomarine sediments, capped with Holocene mud. Till involved in pockmark formation (KELLEY et al., 1994). was not present in all locations and was thickest on The degree of backscatter from the sidescan data was southeastern slopes. Two major unconformities, the basal interpreted as a proxy for bottom sediment type. Areas of erosional unconformity created during a sea-level lowstand, high backscatter, such as gravel and rock, appear dark gray and the ravinement unconformity formed during the latest to black while areas of low backscatter, such as mud, appear transgression, truncate the glaciomarine sediments light gray to white. The surficial sediment type was mapped (BARNHARDT et al., 1997; BELKNAPand SHIPP, 1991). following a sixteen-part scheme with four end members of mud, sand, gravel, and rock and 12 intermediates (BARNHARDT et al., 1996; 1998). Figure 2. Detailed map of the Black Ledges area. The outlined area shows the limits of sidescan sonar coverage detailed in Figure 4. Shaded polygons within the limits of sidescan coverage represent the seven pockmark fields. Line A-A’ is seismic line PB01-26 used in Figure 3. Box A is the location of sidescan sonar data for Figure 5. Box B is the location of sidescan sonar data for Figure 6. Journal of Coastal Research, Special Issue 36, 2002 Sea-Floor Features, Maine 336 Figure 3. Seismic line A-A’. Location of seismic line is shown in Figure 2. Upper panel shows uninterpreted data, lower panel is interpreted. Gas-enhanced reflectors (GER) originate from large areas of acoustic wipeout (NG) suggesting migration along the reflector. Pockmarks, depressions on the seafloor, (PM) occur directly over NG and GER. Other units include till (T), bedrock (BR), glaciomarine (GM) and Holocene mud (M). VE=14x. Figure 4. Sidescan sonar mosaic of the Black Ledges area.
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